Hearing Loop Calculator - IEC 60118-4 Cable Length & Amplifier Sizing

IEC 60118-4 specifies a target magnetic field strength of 400 mA/m (±3dB) at the listener ear height, typically 1.2m seated. This ensures hearing aid users with telecoil (T-coil) can receive a clear, consistent audio signal throughout the looped area. The standard also covers frequency response (100Hz to 5kHz) and signal-to-noise ratio requirements.

A simple perimeter loop suits rooms up to roughly 20-25 metres wide when there is no significant metal present (Ampetronic "Designing Induction Loops" UP60001-13, section 10). You should switch to a low-loss array when metal in the floor or structure restricts the usable loop width (mesh-reinforced or steel-deck floors limit a perimeter loop to about 2-5 m wide), when the room is wider than about 25 m, or when you need to control spill into adjacent rooms — a perimeter loop spills audibly up to four times its width. Low-loss arrays use two interleaved sets of segment loops driven 90° out of phase, giving a flatter field across large or metal-affected areas. The 8 m figure sometimes quoted is a spill-containment rule of thumb, not the perimeter-versus-array limit.

Under the Equality Act 2010, service providers must make reasonable adjustments for people with disabilities, including those with hearing loss. Approved Document M of the Building Regulations requires hearing enhancement systems in reception areas, ticket offices, meeting rooms, and performance spaces. BS 8300:2018 also recommends hearing loops in publicly accessible buildings.

Metal in the floor absorbs the loop’s magnetic field, so more drive current is needed to maintain 400 mA/m. Ampetronic’s Metal Adjustment Table (UP60001-13, section 17) gives typical multipliers for a ~3 m loop or segment width: plain concrete or timber with no embedded steel is ×1.0 (no loss); light mesh-reinforced concrete (≤A142) about ×1.9; heavy mesh (A393+) about ×2.7; a steel deck floor about ×3.2; and a metal or raised access floor up to ×5.6. These are typical figures only — metal losses are very site-dependent, so a survey is recommended, and significant metal usually means a low-loss array rather than a perimeter loop.

The most common cable for hearing loops is 2.5mm² flat copper conductor. For larger installations or long cable runs, 4mm² cable reduces resistance and allows lower amplifier power. The cable should be installed flat against the floor or in the wall, routed to avoid crossing over itself, and kept away from mains cables to prevent interference.